Solidify modifier material Index offset not working
No stupid question. The answer to it is just obvious, if you know it. I struggled with it myself.  :)

Just add another material to your object. In your screenshot it's not visible, if you did this already. The index for Material Index Offset inside your Solidify modifier points to the position of your material in the material list.



So, if you leave Material Index Offset at 0, the newly generated faces will use the same material as the original object - the topmost material in your list. Set it to 1, and all original faces will use original material, while all new faces will use the next material in your list. Same goes for the rim - give it an own material as needed.